6: They're Coming!
The Base Under Siege is a perennial Doctor Who story archetype, so this one gets almost as much detail as the subsets of Historical put together...
The travellers arrive in a smallish, enclosed structure. They get separated from the TARDIS and/or each other. The few residents are besieged by outside forces. The enemy is close to getting in, supplies and morale are both perilously low, there may be a traitor in the camp, something is about to go badly wrong and there's no way out!
Time is no object to The Base Under Siege. It generally happens in the present or a dingy Aliens-style future because that's where most SF Base Under Siege stories happen and they let the locals have guns, but they can happen in the past just fine - my Historical idea could turn into one of these easily, it's already set in a siege.
The Base Under Siege can be part of a larger story, or the entire episode or serial can be played out in the narrow confines of gloomy corridors, enclosed spaces and improvised barricades.
All The Base Under Siege needs is The Base and someone to place it Under Siege. Everything else is optional, but here are some classic elements...
The Base: It could be a space station, a terraforming platform, a modern Antarctic research facility, a WWII submarine, a manor house on a haunted moor, a Roman fort, or a particular nice group of Neolithic cave dwellings. All that matters is that it's remote and isolated, its supplies are limited and its NPC staff small, and it's reasonably well defended for now but how long can it hold out?
Under Siege: Overwhelming forces without. Optionally, also a threat of infiltration and sabotage within. The besiegers could be characterised individually or simply an obstacle like a horde of nigh-mindless monsters. It could also be a natural(ish) phenomenon about to break down the walls and wipe The Base out, like the water if The Base is a sinking ship, since The Base Under Siege as a lot like a disaster movie in a small location. Whatever the case, assuming they're entities rather than oceans, the besiegers want something in The Base. This can be as complicated as vital data from its computer system with which they can threaten a larger goal, or as simple as to eat the people inside it. Individual besiegers might be willing to listen to reason, but the larger force is gearing up to smash puny humans and will not be dissuaded.
A Time Limit: Either there's only so long before the enemy get in, or before the air runs out, or before something else similarly bad happens. And since the time machine is unavailable, that ticking clock can't gain an extra five minutes or hundred years just because it would be helpful. It's impossible to leave, and about to become impossible to stay.
An Escape Route: The Base might have a way out, but it isn't much use at the moment because...
... it's on the wrong side of the wall.
... it will only take so many people. (Currently more than the numbers it would need to take, but give it time...)
... it's broken and can only by repaired by (tech goes here).
... some of the group are trapped somewhere else and would have to be left behind.
Eventually all these barriers will be overcome and the travellers and surviving Base Crew will manage to escape, possibly as part of a desperate final battle against the besieging forces. But the situation should appear hopeless for most of the adventure.
A Small Guest Cast: The Base only holds a few people. This adds to claustrophobia and the sense of overwhelming odds, gives us less people for the GM to introduce and characterise, and keeps the TV budget down. (When the Second Doctor visited Earth's first Moonbase, it had a crew of fifteen. And one was already missing.)
Likely members of the besieged Base Crew include the following. See if you can name them as they appear in The Impossible Planet, The Horror Of Fang Rock, Alien, The Thing or Das Boot.
The Uncertain Commanding Officer: Concerned for his crew, unwilling to put them at risk without a damn good reason, prone to self-doubt. If the travellers hadn't arrived, he'd have to step up and become the hero, or die early on to show that Anyone Can Die. As it is, the travellers are the heroes but he can still prove his worth and/or die to show Anyone Can Die.
The Science Or Medical Officer: Tough, determined, caring. Probably played by a middle-aged character actor, usually a woman to provide contrast with The Uncertain Commanding Officer as his effective equal in rank. Might save a PC's life, might come up with a cure for the virus driving everyone mad, might go mad. Likely to survive, because this is Doctor Who and scientists and medics are laudable professions.
The Security Chief: Well-armed, gung-ho, sometimes Plagued By Doubts. Likely to die late, probably in a heroic last stand.
The Tech: Young, cocky, able to fix the device that could save The Base or destroy it. Unlikely to appear in historical settings as there are less high-tech devices to explain to the Companions and audience. Likely to die in an explosion, possibly of their own making.
The Traitor: Working for the besieging forces or against the group in The Base for another reason. Generally appears to be one of the others on the list. Optional, usually to be used if the besieging forces can't talk so the heroes have someone to talk to. Bound to die in or around the final battle or escape, probably ironically.
The Added Problem: May be an addition to one of the above stereotypes, or a separate character. The Added Problem has a heart condition, a broken leg, failing vision, a small child in tow, a drinking problem or something else that would slow up a full-tilt dash to the escape route. This may kill them, kill someone else who goes to help them, make them or someone helping them step up and become the hero, be completely coincidental in them being killed, or be completely coincidental in them surviving.
The Shreddies: Backups for most of the above, spare soldiers, pilots, experts in whatever The Base was supposed to be doing which doesn't really matter any more and other assorted extras. There to show us how the monster works, get a bit of characterisation and then show that Anyone Can Die, turn out to know or do something vital and then optionally die immediately afterwards, be killed by The Traitor to reveal that there is one or who it is.
Or, if the PCs get really attached to one, they might survive. One could even be a player's potential Companion character who happens to have no skill or role particularly useful in the siege but should hopefully get along well enough to be invited to join the travellers at the end of the adventure. Of course, if they're the only survivor that might affect their decision too...
Scenery Details: The Base can have large, impressive areas but these are hard to defend and likely to fall early. It must have small, cramped environments. Ideally, it should have corridors to run up and down, crude improvised barricades (or proper purpose-built barricades) to hide behind, ventilation shafts or sewage systems to crawl through, not-that-big central areas that are good places for a last stand when the besieging forces break into The Base, woefully inadequate medical facilities, a view of the outside to establish how remote and isolated The Base is, somewhere for The Uncertain Commanding Officer to brood over lost comrades, and a not-currently-functional Escape Route.
Atmosphere: The Base is gloomy, run-down, lived-in, claustrophobic, smoky, dank... Of course, it could also be lovely as a contrast to the terrible Things outside, but if The Base has been in use for long living conditions are likely to go downhill.
Floorplans: I'm normally averse to using maps in RPGs because they can tie down the action and discourage crazy stunts. But in The Base Under Siege, knowing what goes where can be very important, as well as leading to tense scenes where everybody looks at the floorplan and discusses whether they can make it through the ducts to Section 3 in time. However, it doesn't have to be, as even the most claustrophobic Doctor Who story is still a Doctor Who story, and a game with Story Points can easily have situations where it "turns out" that the fuel silo is nearer to the habitation ring than might otherwise have been the case. (As an example from the most claustrophobic Doctor Who story, the stewardess in Midnight obviously spent her one Story Point to have Sky standing so close to the blast door. :D )
A Twist: This is optional, can be provided by The Traitor or the besiegers or some other source. It turns out, around the final escape, that things aren't quite as they seem. Be careful not to make this change things so much that the players' efforts on behalf of their characters seem like a waste of time. If it turns out that the besiegers are peacefully trying to reclaim a sacred object taken by an archaeologist and currently used as a paperweight in the Commander's office, the players will probably be a bit narked. If it turns out that the besiegers don't know they're endangering The Base, it could still feel like a worthwhile adventure if the players' characters manage to get them to stop peaceably.
Two published adventure examples, both involving sea monsters threatening a near-future submarine, with very different rules systems.
Grace Under Pressure (Jeff Barber and John Tynes, for Call Of Cthulhu) runs through a number of the clichés of The Base Under Siege in a (when-published) futuristic deep-exploration sub under attack by terrible Things. It provides isolation by mapping out the eight-strong sub in precise detail so you can see just how cramped it is.
Grave And Watery (Robin D. Laws, in Pantheon And Other Roleplaying Games) looks at the same situation in a narrative parlour game RPG where the players introduce plot elements even as their characters struggle against them, and score points for the ideas they bring to the table. Having your character die in a suitable manner could win you the game. Even if you never play the game itself (and knowing what the scoring plot points are would spoil it) it's a great checklist for The Base Under Siege.
(Yes, the specific setting is totally influenced by the above examples.)
The TARDIS lands inside a submarine terraforming base on an isolated planet, threatened by robotic drones sent by a rival power and by a saboteur inside the base. The surface pods were the first things to be wrecked, and while some could be repaired, someone would have to go outside to do so... and some of the crew families have reported strange lights in the water outside... and the air filtration system has been ruptured and is about to explode...